Sound recording apparatus



lwcnvtor Homey AAAAA SOUND RECORDING APPARATUb Filed oct. 24, 1940 Dec. 22, 1942.

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2,305,861 SOUND mscoanmc APPARATUS John B. Gehman,

Drexel Hill, Pa., assignor to Radio Corporation of America, a corporation of Delaware Application October 24,

1940, Serial No. 362,581

10 Claims. (Cl. 179-1003) McDowell patent by interposing a shutter in theI recording light beam and causing the shutter to follow the envelope of the sound waves being recorded so as to leave a corresponding transparent area on the negative. .This sound record when printed o'nto the sound record produces a corresponding opaque area. In the recording of variable density sound records a corresponding procedure has been used by varying the opening of the light valve or intensity of the glow lamp as the case may be, so that at low sound intensities theaverage light transmitted to the negative is comparatively little, increasing with the sound amplitude and thereby increasing the average density of the negative sound record with amplitude and conversely reducing the average density and light transmission of the positive sound record at low amplitudes.

The amplier shown in the McDowell patent was reasonably satisfactory for the purpose but improvements in the standards of quality of sound recordingand reproduction have been so noise reduction device is opened by a predetermined amount and stays at that opening for a predetermined time unless a signal of greater amplitude should follow within that time and cause further opening.

This is accomplished by the use of a series of I gas tubes which operate at different potentials.

great that improvement in the ground noise reduction amplier has been continually required. One deficiency of early types of amplifiers has been that they tended to follow .the wave form of the sounds being recorded too closely. Another disadvantage has been that when a sound consisted of a series of peaks spaced at intervals large in proportion to the amplifier tended to clip the peaks. Another disadvantage has been that with the more usual types of amplifiers it has been diicult to provide a device which close slowly. In an ideal type of ampliiler the .amplifier should open as rapidly as required without producing thump and should maintain the opening level for a predetermined time after cessation of the signal before starting 'to close as described in Reiskind application Serial No. 361,835,1i1ed Oct. 19, 1940.

'I'he present ampliiier is an improvement on that of Reiskind in that, on the application of the timing of the peaks would open rapidly and an improved amplifier a signal within predetermined limits, the ground 55 The use of gas tubes operating at different potentials tosecure a step by step control is broadlyshown in'Strieby Patents Nos. 1,776,821 and 1,776,822, issued September` 30,` 1930. In those tubes are used for the purpose which require the application of high voltages to secure their operation, and the currents passed through the tubes are used to actuate the controlled mechanism.

In the present device, I provide an arrange? ment of gas filled hot cathode tetrodes which are used to control the vacuum tube amplifiers supplying operating current 'to' the ground noise reduction shutter. With this of the amplitude of the currents controlled and they may be set to operate at much lower voltages than required by the tubes of the Strieby patents so that a much greater range of volume may be controlled. The amplifier tubes connected to the successive gas tubes are arranged to control an improved type of direct current amplifler which actuates the ground noise reduction apparatus.

One object of the invention is to provide an improved type of ground noise reduction timing circuit.-

Another object of the invention is to provide an improved control circuit which is capable of any required adjustment as to time constants.

Another object of the, invention is to provide having an A. C.input and a D. C. output.

vOther and incidental objects of the invention will be apparent to those skilled in the art from a reading of the following specification andan inspection of the accompanying drawing in which the single gure shows the preferred form of the invention. -l

The speech amplifier is illustrated in the upper left portion of the figure and is of more or less conventional type, imum outputvoltage of approximately 300 at maximum input corresponding to 100 percent modulation of the sound track. In this amplifier, the speech input is applied across the 600,000 ohm potentiometer I0, vwhich determines the portion of the input signal applied to the control arrangement, the gas tubes may control the timing independently but is designed to give a maxgrid of the tube II which is preferably of the commercial type known as the 6J7. 'I'his tube is provided with a cathode resistor` I2 having a value of approximately 470 ohms and with a screen dropping resistor I3 of v1,200,000 ohms. The 220 volt positive potential is supplied to the screen and 'plate through the 15,000 ohms resistor I5 and is fed to the plate through the 220,000 ohm resistor I4. An appropriate by-pass condenser of mfd. capacity 23 is provided. The output from the tube II is applied to the tube I9 through the 0.1 the 390,000 ohm resistor I8.

'I'he tube known as the GF6 is provided with the usual cathode biasing resistor of 680 ohms. Degeneration is provided by feedback 'from the plate of the tube I9 through the resistor I5 having a resistance of 150,000 ohms and the filter composed of the and the capacitor Il having a value of .025 mid. The output of the tube I9 is fed through the transformer 2| to the input. of the step amplifier. Direct current at a potential of 220 volts is supplied through the power supply terminal 60 and 6I. 'I'he outputl from the speech amplier is applied across the lve potentiometers in the iive stages of the step amplifier. It will be apparent to those skilled in the art that a greater or lesser number of stages may be used according to the number of steps required, and it will be also apparent that instead of five similar potentiometers, as indicated in the drawing, each having a value of .5 megohm, appropriate potentiometers of lower value may be used connected with series resistors so that the potentials applied to the successive tubes may be precisely adjusted. ABetween the potentiometer 3U and the gas tube 32 there is provided a 1 megohm protective resistor 3| so that unusually high signals will not injure the tubes and so that a maximum signal will not injure the tube adjusted to respond to the minimum signal.

The tubes 32 are preferably gas tetrodes of the type commonly known as. the RCA 2051.

30 are so adjusted in the successive stages that this voltage is applied at the appropriate volume level in the recording apparatus.

The second grid of each of these tubes is connected to the cathode as o1 each tube is connected through the resistor 34, and the plate of the tube 32 and the grid of the tube 35 are-connected together. The resistor 34 preferably has a value of approximately 330,000 ohms and the capacitor 33 may have avalue of the order of .05 rnfd. in order to secure the time of response which is, at present, considered most desirable, that is, a

Aclosing or release time in the output of aptiee increase in plate current supplied by each s p.

mfd. condenser 24 across I9, which is preferably of the typev resistor I6 of 120,000 ohms the plate of each -tube preferably that commercially 991s and each provides When no signal is applied to the apparatus, the potential in the capacitor 33 is built up to its maximum and all of the tubes 35 are biased to their normal current of -5 volts. When a signal is applied to any stage sufficient to trigger the tube, the tube 32 immediately. becomes highly conductive and the capacitor 33 discharges com.- pletely through the tube 32 at an extremely high rate. This promptly raises the negative grid potential on the corresponding tube 35 considerably beyond the cut-off of the tube 35 and causes a corresponding opening of the around noise reduction shutter or other device connected to the output of the shutter amplifier shown in the 1 upper right hand portion of the figure. When the capacitor 33 has discharged.to a potential corresponding to the minimum plate voltage on the tube 32 which is approximately 14 volts the potential on the grid of the tube 32 again takes control and the tube becomes nonconductive. Potential then builds up on the capacitor 33 through the resistor 34 until either the capacitor 33 is charged to maximum potential or a subsequent signal'has again triggered the tube. It will be apparent from this that each stage, including the tube 32 and the tube 35, constitutes a separate saw-tooth oscillator triggered by the input signal and having a periodicity determined approximately by the capacitor 33 and the resistor 34. In this circuit, as in mostA saw-tooth oscillators, the frequency is varied to a certain extent by the frequency of the applied signal. If a signal of slightly varying level approximating the level for which any .one stage is set were applied, there would be a tendency for that stage to turn on and o1! irregularly, and this is prevented by the portion of the circuit next described.

The power supply of 220 volts is applied a`t the terminals indicated. The tubes 41 and 48 are of the type known commercially as the RCA a voltage drop of volts. These tubes limit the potential difference between the terminal 60 and the lead 49 to 120 volts, leaving a potential of volts distributed across'the resistors 4II, 42, 43 and 44 between the lead 49 and the terminal 6I. Similarly, the

tube 46 which is preferably of the type known as an RCA VR-150, which is a unidirectlonally conducting tube establishing a voltage drop of volts, limits the potential between the midpoint of the resistors 40 and 42 and the lead to the cathodes of the tubes 35 to 150 volts, leavlng the balance of potential of 220 volts or 70 volts distributed between the 40 and the 29,000 ohm resistor 45. Due to this arrangement, when one of the tubes 35 becomes conducting and plate current starts to flow, since the Vpotential difference between the resistor 4l and the cathode lead of the tube 35 is maintained constant at 150 volts, an increase'in the .f drop amounting to approximately 1/2 volt occurs Y across the resistor 40 for each stage triggered, thereby decreasing the'grid bias applied to the tubes 32 by a corresponding amount and causing the tubes32 to remain conductive as long as the input signal does volt below the signal-which originally turned on that particular stage. Conversely., whenthe input signal drops more than one-half volt below that which originally turned on the stage, that stage'becomes inoperative and remains so until the signal reaches the original trigger level.

As described above, the five stages of the step l ampliers A, B, C. D and E may be made identi- 68 ohm resistor not fall more than one-half level of -60 db. may be for: Decibels Stage A -50 Stage B -35 Stage C -20 Stage D- Stage E --4 If potentiometers of smaller range with series resonators be substituted, satisfactory ranges are for:

Decibels Stage A -50 to -44 Stage B -40 to -30 Stage C -32l to -15 Stage D -22 to maximum Stage E -10 to maximum Since each of 'the stages of the step amplifier is, as pointed out above, substantially a sawtooth oscillator, there is a tendency for the device to produce a buzz which is applied across the resistor 42 and thereby applied to the untriggered stages through the condensers 33 of each oi them. The untriggered stages transmit the buzz to the ground noise reduction apparatus.

This buzz may be eliminated from the outputby the provision of a bypass condenser 4i. This condenser should have a capacity of not less' than 2 mid. and a condenser of 10 mid. is preferable. y

As pointed out above, each of the tubes 35 is biased considerably above cut-off when the corresponding tube 32 is triggered. Due to this increased bias, the tube 35 does not begin to conduct until the charge on the corresponding capacitor 33 has built up to an appropriate value through the resistor 3i.- The tube 35 will -then gradually become conductive following an exponential curve determined by the charging rate of the condenser 33 through the resistor 34. Due to this arrangement, the output curve of a single stage will remain flatfor a length of time determined by the time required for the capacitor 33 to reach the cut-ofi potential of the tube 35 from the relativelyl greater negative potential applied to the grid of tube 35 and, after reaching this point, ponential .curve in becoming conductive. The timing of the exponential curve may be varied by changingn the capacitor 33 or the resistor 3 4,l

or both, while, with any given timing for this time circuit, the length of time during which the tube remains non-conductive after cessation of a signal is determined lby the excess of negative voltage applied to the grid of the tube 35. With this arrangement, if the shutter is opened to a'given amount and the signal suddenly decreases and thereafter ,immediately increases to its original value within level, the shutter will not change in its position.

The output from the step amplifier is fed to the shutter amplier illustrated in the upper right hand portion of the figure. Although this is called a shutter amplier, it will be understood that it can be used'to operate either a ground noise reduction shutter such as referred the tube 35 will follow an ex' 'open very rapidly,

to above or a shutter such as in variableV density recording, or the output may be applied in an appropriate manner to any other type of ground noise reduction apparatus, volume compressor, volume expander or any equivalent device. Although the output from the step amplifier might be suliicient to operate the shutter or other device, the shutter ampli- 'fier is necessary in order to-isolate the output circuit from the thump filter next described, as otherwise the inductance, etc., of the output circuit will form a resonant circuit with the capacitor 5| of the thump filter.

In this amplifier, the output from the step amplifier is applied across the potentiometer which is bypassed bythe of approximately 3,000 ohms and with the capacitor 5| constitutes a thump' lter. As pointed out above, the several step amplifier stages practically at radio frequency. In the applicationof the output of the step ampliiier to ground noise reduction apparatus, it is necessary to slow this action down to an inaudible frequency, as otherwise the step action of the amplifier will be reproduced as a clicl:-

or thump. The particular lter arrangement shown permits an opening time of approximately 20 milliseconds, which is below the normal ire-` quency range transmitted by present theatre' amplifiers or reproduced by the present commercial types of speakers. Since the output from the step amplifier is direct current and at a fairly high positive potential, it is necessary to provide an appropriate potential source 53 between the contact to the potentiometer 50 and the grid of the tube 53. This source of potential is indicated as a variable battery which, of course, may be used, but it may be preferable to use rectified high frequency current instead of a battery. The tbe 53 is 'preferably RCA type GF6 and may be provided with an adjustable cathode bias resistor 54 of 1500 ohms resistance. The resistor 56 may havea resistance of approximately 20,000 ohms and the tube is preferably the RCA type known as the VRf-l50 which provides a drop of 150 volts. It will be apparent that the tube 53, the tube 55,- the resistor 54 and the resistor 56 form a bridge cirthe resistor 50, thereby decreasing the bias on the tube 53 and increasing its plate current.

In current commercial practice, the load resistance of aground noise reduction shutter such as indicated at 51 is approximately 100 ohms the time of constant and it requires a maximum current of 30 milliamperes. When the apparatus is properly adiusted, the resistor 5l isso adjusted that there is at least ,30 milliamperes current through the load when the tube 53 is biased to cut-ofi. There is 150 volts drop across the tube 55 with a currentof 35 milliamperes through it and 5 milliamperes of current passing through the resistor 56 to keep the Atube 55 in operation. With a. supply current of 220 volts there is a. rI0 volt drop across the resistor 56 and a 21 volt drop across the '100 ohm load 51. The voltage drop acrossthe tube 53 is then 171 volts and that across the resistor 54 is 49 volts; When the circuit is thus connected, the control grid of the tube 53 becomes more positive on increase in output from commercially used 1,5 mfd. capacitor 5i. AThis potentiometer preferably has a resistance gradually increases until the speech amplifier and draws current since the voltage across the tube 55 remains constant at 150 volts. As the current drawn by the tube 53 increases, that permitted to pass through the load 51 decreases practically any desired amount.

If it is desired to use this circuit to operate a variable density shutter wherein the shutter current is required to increase with increase in signal current, the shutter place of the resistor 54 and the tube 55 is removed. In this arrangement, the increase in current through the tube 53 causes a corresponding increase in current through the load 51.

It will be apparent to those skilled in the art that the invention is not limited to the exact construction before described but that the construction and characteristics of the apparatus may be greatly varied within the scope of the invention. IIn the apparatus above described, if a gradually increasing signal is applied to the speech amplier, the step ampliiier will increase its output step by step as -the increasing signal the maximum output is reached. If the signal is then completely removed all the timing circuits which are similar cause the output of the several stages of the will step ampliiier to decrease simultaneously, and the output will drop to zero at the time determined by any one of the timing circuits. If the signal decreases gradually, the output will de- 'crease step by step at the end of the closing time determined by the timing circuit of each step.

These timing circuits need not all be made simi-- lar, but they may be made to operate on different times; could be decreased from stage A to stage E making the larger capacitor on the rst stage and the smaller capacitor on the last stage so that on the sudden cessation of a maximum signal. the output would decrease step by step leaving the ground noise reduction apparatus partly open fora longer time to receive a signal ofsmaller amplitude. l

The characteristics of the opening timing circuit 50, 5I may be varied as desired according to the characteristics of the apparatus with which the amplifier is to be used, but it should be kept in mind in making any such variation that the ground noise reduction apparatus should open as rapidly as possible without producing any audible thump.

lHaving now described my invention, I claim: 1. In combination, an amplifier' having an A.C. input, a D.C. output, means for increasing the D.C. output step-by-step on a gradual increase in A.C. input, means for causing a gradual decrease in D.C. 'output on cessation of A.C. input, and a D.C. ampliiier connected to said last-mentioned means.

2. In combination, an amplifier having an A.C. input, a D.C. output, means for increasing the D.C. output step-by-step on a gradual increase in A.C. input, means for causing a gradual decrease in D.C. output a. predetermined time after cessation of A.-C. input, and a D.C. amplifler connectedtosaid last-mentioned means. 3. A ground noise reduction amplier including a speech input amp er, a step amplier having speech input and D.C. output, and a D.C. amplifler connected to the output of said step amplimay be inserted in For example, the values of the capacitorsv -timing circuits controlling bridge,

fierand adapted to actuate the ground noise reduction apparatus.

4. A ground noise reduction amplifier including a speech input amplifier, a step amplifier having speech input and D.C. output and including the rate of decrease of output, and a D.C. amplier connected to the output of said step amplifier and adapted to actuate the ground noise reduction apparatus, said D.C. ampliiier including a timing circuit controlling the rate of increase of output.

5. In a trigger type ampliiier, a gas-type trigger tube including a control grid, a'vacuum type amplifier tube, means for applying a plate potential to said trigger tube and a grid potential to said amplifying tube, means for applying a grid bias to said trigger tube, and means for decreasing said grid bias on the triggering of said tube.

6. In a trigger type amplier, a gas-type trigger tube including a control grid, a vacuum type amplier tube, resistance means for applying a plate potential to said trigger tube and a grid potenapplying agrid bias to said trigger tube', and resistance means for decreasing said grid bias on the triggering of said tube.

'7. Ina trigger type amplifler, a gas-type trigger tube including a control grid, `a vacuum type amplier tube, resistance means for applying a plate potential to said trigger tube and a grid potential to said amplifying tube, resistance means for applying a gn'd bias to said trigger tube, resistance means for supplying plate `current to said vacuum tu-be, and resistance means for decreasing said grid bias on the triggering of said tube, said last two resistance means being connected whereby increase in said plate current causes said decrease in grid bias.

8. A D.C. output amplifier including a vacuumv type amplifying tube forming one arm of a a Voltage regulating tube forming a second arm of the bridge, resistors forming the v other two arms of the bridge and normally balancing the said bridge, and an output circuit connected across said bridge.

9. The method of producing a photographic lsound record of the noiseless type comprising exposing a sensitive illm to a beam of light, varying thev beam of light in accordance with the wave form of the -sound being recorded, rapidly increasing the average quanta of the beam upon the initiation of sound waves of any amplitude to correspond to the peak values of said sound Waves of any duration, and causing the average quanta of the beam to be reduced upon the reduction of a sound wave at 'a rate dependent both upon the duration and peak amplitude of said sound waves.

10. A noise reduction system comprising a speech input amplifier, a direct current amplifier adapted to be connected to the ground noise reduction apparatus, and means lconnected intermediate said speech input amplier and said direct current amplifier for producingactuation of said ground noise reduction apparatus in accordance with the peak values of soundwaves of any duration.

v JOHN B. GEHMAN. 

